1. Field of Invention
This invention relates to controlling operation of dragline equipment of the type employing an extended boom from which a bucket is suspended by means of a hoist rope or cable and a drag rope or cable that are attached to the bucket and either paid-out or taken-up by respective hoist rope and drag rope winches. By appropriately positioning the boom and either paying-out or taking-up the respective hoist and drag ropes, an operator of the equipment causes the bucket to excavate soil or other material from selected locations to a desired depth, or do other similar work.
More particularly, the invention relates to a new and improved anti-tightline control system and method for automatically overriding the operator's setting of the controls for dragline type equipment in order to avoid tightlining the equipment. The novel control system and method also simultaneously signals the operator that a tightline condition is imminent and also, if necessary, serves to shut down the equipment automatically before a threatened tightline condition can occur.
2. Background Problem
FIGS. 1 and 2 are schematic illustrations which depict typical dragline equipment and the environment in which such equipment is used and are useful in defining the problem to which the present invention is directed. The dragline type equipment is comprised by a cab 11 mounted on treads or otherwise so that the cab can be moved about on the ground from one location to another for excavation or other similar work which needs to be done. Attached to the base frame of the cab is an extended boom 12 which can have lengths up to and exceeding 300 feet, for example. Supported from the end of the boom 12 is a bucket which can be raised or lowered in the vertical direction by a hoist rope 14 that extends upwardly from bucket 13, along the length of boom 12 and to a rotatable winch, spool or drum supported within cab 11. The drum is driven by a motor for paying-out or taking-up the hoist rope 14 thus lowering or raising bucket 13. Also attached to the bucket 13 is a drag rope 15 which also extends to a rotatable winch, spool or drum on cab 11 that is driven by an electric motor for causing the drag rope 15 to be paid-out or taken-up thereby causing bucket 13 to move away from or toward the cab 11. By appropriately rotating the cab 11 and hence boom 12 to a desired angle and dropping the bucket 13 to the bottom of a hole or pit being excavated, and thereafter drawing in the drag rope 15 to fill the bucket 13 to a desired degree and subsequently raising the bucket via the hoist rope and rotating the cab around to a desired deposit point, the dragline type equipment accomplishes its work of excavating the pit. In the arrangement shown in FIGS. 1 and 2 the dragline 11 is positioned at a high point on the land adjacent the pit being excavated (referred to as the bench) and the bucket 13 lowered, dragged, raised, rotated to deposit, and then rotated back in the above briefly described cycle. Other arrangements can be visualized when the dragline is situated on a bench below a bank to be removed, etc. However, the arrangement of FIG. 1 is believed to suitably illustrate a typical situation wherein a tightlining problem to which the present invention is directed, can arise.
It will be appreciated by the reader that while FIGS. 1 and 2 illustrate a particular type of dragline wherein an actual bucket is moved to perform excavation of earth or other similar materials there are other types of equipment such as cargo cranes and the like which employ a large holding magnet, a claw, a platform or other device for performing work. Thus in the present specification the term "dragline type equipment" has been employed to identify all such equipment wherein a tightlining problem might arise and the term "bucket means" has been employed to identify all such devices as a bucket, magnet, cargo platform, etc. Further, while in the illustrations of FIGS. 1 and 2, hoist and drag ropes have been described, it is believed equally obvious that the hoist and drag ropes could comprise hoist and drag cables, hoist and drag chains, hoist and drag lines, or other similar items which could be employed in place of the rope and hence the term "rope means" has been employed to encompass all such similar items.
From a consideration of FIG. 1 it will be appreciated that a given amount of hoist rope on or off the hoist rope winch drum represents a given bucket height measured from the tip of boom 12 if the bucket is hanging vertically. If the bucket is dragged in from the solid line position to the dotted line position shown in FIG. 1, there will be a certain length of drag rope on or off the drag rope winch drum as the bucket 13 is made to depart from the vertical below the boom tip. This dragging in of the bucket causes an increase in compressive stress and an increase in bending moment on the boom 12. If thereafter, continued operation of the hoist rope winch and/or the drag rope winch in the take-up direction causes these stress levels to exceed certain design limits established for the boom by a manufacturer of the dragline equipment, such increased stress levels beyond the design limits can be detrimental to the boom system. This detrimental condition is called tightlining.
In addition to the tightline condition described in the preceding paragraph with respect to FIG. 1, which will be referred to hereafter as a static tightline condition, a further condition can be brought about whereby the dragline bucket 13 is caused to collide with the boom. This shocking of the boom condition will be referred to as a dynamic tightline condition. A dynamic tightline condition can occur if the bucket velocity is of such a magnitude that it is virtually "thrown" into the dragline boom. This can occur if the angular relationship, indicated as the angle .phi. shown in FIG. 2, between the hoist and drag ropes approaches or exceeds 180.degree.. It is possible by means of the present invention to detect that a boom collision is imminent by using drag and hoist rope lengths and speeds. Such a detected condition can then be used to prevent or warn against any further operator action which would worsen this condition. This is called a dynamic anti-tightline control feature which when coupled with the ability to detect and limit bucket position so as to avoid a static tightline condition as described above with relation to FIG. 1, provides a preferred form of anti-tightline control system for avoiding both static and dynamic tightline conditions.
The reader will obtain a better appreciation of the need for an anti-tightline control system according to the invention from a consideration of FIG. 3 of the drawings which illustrates a family of limit curves for various hoist and drag rope speeds 5 seconds prior to boom collision. In FIG. 3, the boom 12 is indicated to be 300 feet long and disposed at an angle of 30.degree. relative to horizontal. In the vertical scale, the distances measured are relative to distance above and below the bench. The five elliptically shaped limit curves are for different combined lengths of hoist and drag rope under conditions where either the hoist or drag rope or both are being taken-up at the speeds noted. The speeds are noted in parts per unit (p.u.) where 1.0 p.u. speed equals 15 feet per second. Thus, a speed of 0.5 p.u. would correspond to a speed of 7.5 feet per second.
In considering FIG. 3, one should keep in mind the practical problem confronting operators of dragline type equipment. In the situation depicted in FIGS. 1 and 2, the bucket 13 when loaded must be lifted above the bench, the cab and boom rotated to the deposit location and thereafter rotated back and the bucket dropped to complete an excavation cycle. For maximum efficiency of operation, it is not unusual for an operator when the bucket is at the bottom of the pit, as shown in FIGS. 1 and 2, to raise the bucket at a maximum speed to the point above the bench where the boom can be rotated towards the deposit location. From a consideration of FIG. 3, it will be seen that if both the hoist and drag ropes are taken-up at 1.0 p.u. speed the limit curve 5 seconds before boom collision occurs at the point when there is still 460 feet of combined hoist and drag rope paid-out. If only the hoist or only the drag rope is taken up at 1.0 p.u. speed then the limit is at 380 feet of combined hoist and drag rope. Such a situation is very difficult for even an experienced operator to visualize and react to even if he is provided with input measurement signals which convey the above noted information to him.
From the foregoing brief description, the reader will appreciate that not all operating phases of dragline type equipment can give rise to either a static or dynamic tightline operating condition. The situations wherein tightlining can occur are listed in the following anti-tightline logic table together with an indication of what actions a well designed anti-tightline control system should provide.
______________________________________ ANTI-TIGHTLINE LOGIC TABLE Result with Drag Hoist Con- Trip-Out Result with Function Function dition System Regulating System ______________________________________ 1. Pay out Lower Tightline No corrective not action required possible 2. Pay out Neutral Tightline No corrective not action required possible 3. Pay out Hoist Tightline Trip-Out Reduce hoist speed 4. Drag in Lower Tightline Trip-Out Reduce drag speed 5. Drag in Neutral Tightline Trip-Out Reduce drag speed 6. Drag in Hoist Tightline Trip-Out Reduce hoist speed 7. Neutral Lower Tightline No corrective not action required possible 8. Neutral Hoist Tightline Trip-Out Reduce hoist speed ______________________________________
The present invention provides an anti-tightline control system and method which includes the ability to detect and limit bucket position to avoid a static tightline condition as described with relation to FIG. 1 and also the ability to detect and limit bucket position and velocity in order to avoid a boom collision (dynamic tightline condition) as shown in FIG. 2. The limit function is not restricted to a warning and trip-out type of system but also includes the functional means to cause a control regulating action to occur. The regulating action takes over control of the dragline equipment and slows down or ultimately stops the drag and/or hoist motors for logic conditions indicated as 5, 6 or 8 of the logic table set forth above. For logic conditions 3 and 4, this regulating function reduces and ultimately stops the drag or hoist motion drive motor. In the case of pay-out of the drag rope and/or lowering of the hoist rope, these functions are not affected since neither can lead to a tightline operating condition. All of the above suggested limiting actions will cause the bucket to traverse along the dynamic tightline limit curve, even though an operator signal would otherwise cause a tightline condition to occur. The word tightline as used in the above logic table as well as hereinafter in this disclosure, should be construed to mean both static and dynamic tightline if neither one is specified.